The invention is related to a method for recycling iron-bearing dusts and sludges back into a steelmaking process, and more particularly, it relates to a method for recycling iron-bearing waste materials back into a basic oxygen furnace (BOF) simultaneous with the oxygen blow used in the process of converting molten iron into steel.
It is well known in the art of steelmaking that iron-bearing dust and sludges, generated by steelmaking furnaces, are valuable revert materials suitable for recycling back into steelmaking operations. Such waste materials contain iron oxides in an amount up to about 50% by weight, and it is very desirable to recover the iron for use as a steelmaking charge material. However, in the case of wet sludges, and especially in the case of wet BOF scrubber sludge, high moisture content makes the wet sludge very difficult to handle in a recycling stream.
Filter cake produced from wet scrubber sludge typically has a moisture content of about 30% by weight. The high viscosity of such wet sludges causes them to have poor handling characteristics. They stick to conveyors and machinery when attempts are made to convey them as reverts in a recycling stream. They move poorly and often form sticky agglomerations that clog and shutdown equipment and machinery. For example, under test conditions, it has been found that wet sludge having a moisture content of &gt;10% has a flow rate of less than 10 pounds of sludge per minute. Such low flow rates make wet material very unsuitable for recycling as a steelmaking revert.
In instances where waste steelmaking dust is recycled back into the operation, the dry powdery condition of the material causes environmental dusting problems. To control dusting either water is added or the dry material is mixed with wet sludges to eliminate the dusting. However, when various different wet or dry waste materials are combined to produce a steelmaking revert, high levels of undesirable elements and compounds can be introduced into the steelmaking process. For example, if hot dip coating sludge is introduced into the recycling stream, the zinc in the recycling stream can rise to a level where the waste material is unsuitable for use in a steelmaking furnace. Therefore, such combining of steel plant wastes must be carefully monitored for chemistry to avoid introducing deleterious elements into the steelmaking process.
Various apparatus and methods have been developed in the past to reduce the moisture content and/or recover iron from wet sludges. For example, United States Reissue Patent No. 30,060 teaches a process that instantaneously vaporizes the water in sludge by spraying the sludge into a hot (1200.degree. F.) gas stream. U.S. Pat. Nos. 4,091,545 and 4,133,756 also teach using a hot gas to reduce the moisture content of wet sludge.
U.S. Pat. Nos. 5,114,474, 4,725,307, 4,711,662 and 2,710,769, teach mixing wet sludges and dust with molten slag to produce reverts. The mixture is crushed for recycling after the slag cools and solidifies.
An article in "33 METALPRODUCING," March 1997, discloses a process that forms BOF waste sludge into briquettes. The apparatus used in the process includes a rotary kiln or dryer to remove water from the sludge, a roll-press, screw conveyors and pug mills. Such recycling plants require large capital investments. The use of a rotary kiln consumes expensive energy to generate heat for drying the sludge. The article also discloses using heated molasses as a binder to form the briquettes. The heated molasses also adds cost to the recycling process.
In the February 1997 issue of "NEW STEEL", an article by John Schriefer entitled Reaping the value from dust and slag, discloses that 11 steel mills located throughout Indiana and Ohio generated 959,000 tons of wet BOF sludge during the year 1992. In an effort to reduce landfill costs associated with disposing of such sludge, and in an effort to recover the valuable metallic materials contained in the sludge, such sludge is processed into briquettes for recycling back into a BOF as a source of iron. The article further mentions that such iron-bearing briquettes replace some of the steel scrap used in the BOF steelmaking process. Therefor, from a steelmaking viewpoint, it would appear reasonable that the article teaches charging the iron-bearing briquettes into the BOF vessel prior to the molten iron charge.
U.S. Pat. No. 3,870,507 to Allen is also directed to forming iron oxide briquettes. However, the Allen briquettes are recycled into a blast furnace with new iron making materials. Steelmakers are reluctant to charge such briquettes, dusts and sludges, back into steelmaking furnaces because steelmaking wastes usually contain high levels of heavy metals, for example zinc, and they adversely affect the quality of the finished steel product.
Finally, U.S. Pat. No. 5,496,392 to Simms, et. al. is directed to a method of recycling industrial waste. The recyclable iron-bearing material is either pelletized or formed into briquettes. The patent teaches that such briquettes are charged cold into a steelmaking furnace and heated up to temperature. The patent is absent of any teaching that would suggest charging such iron-bearing reverts into the vessel simultaneous with the oxygen blow.
Therefore, as shown in the patents, it is well known within the steelmaking art that steelmaking sludges and dusts can be processed and recycled as valuable reverts. It is also well recognized that wet sludges flow poorly and create logistical problems within a recycling stream because of their poor flow rate properties. The patents also teach that dry steelmaking waste materials present dusting problems during recycling. Finally, the above patents teach solving these well-known problems by dehydrating wet sludges with hot gases to produce a suitable sinter or charge material. The current state-of-the-art requires complex recycling facilities and hot gas blowers that consume large amounts of expensive energy to dry wet sludges. The hot gas vaporizes the water in the wet sludge and reduces the moisture content of the sludge to a level where the sludge can be used as a steelmaking revert.